[Pw_forum] vc-relax : total force does not converge

[Lorenzo Paulatto]

On 5 September 2012 10:22, Jiseok Kim <tube77 at gmail.com> wrote:

> Yes. I checked the geometry before and after the 'relax' which looks ok ...
> The atomic distance between neighboring atoms about 12A along x-direction
> and about 24A along y-direction..
> The vacuum region along x-direction might not be large enough but I
> believe it wouldn't be a problem having considered that the 10~15A of
> vacuum region usually is accepted. (Please correct me if I'm wrong.)
>

I think your forc_conv_thr is just too low, if you want to reach that
ridiculous precision you need to reduce the conv_thr and/or increase
upscale. I have a couple more observations:

1. why are you using "cg" diagonalization? If the electronic calculation
does not converge with the default algorithm, there is probably a problem
in your input; changing algorithm will just mask it.
2. why are you mixing ultrasoft (Si) and norm-conserving (H)
pseudopotentials? It is not forbidden, but you get the worst of both
methods: high wfc cutoff (NC) and high charge density cutoff (US).
3. does it make sense to do vc-relax in a isolated 1D system? Can't the
system just rotate if it wants more space?

bests regards, and good work

-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
phone: +33 (0)1 44275 084 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
mail:  23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05
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